Medical devices such as intravascular filters, guidewires, catheters, needles, needle stylets, and stents are used in performing a wide variety of medical procedures within the body. Such devices are typically formed from a number of components each exhibiting a differing performance characteristic within the body. In some medical applications, it may be desirable to alter the flexibility characteristics of certain components to improve the function of the device within the body. In the field of embolic protection filters, for example, it may be desirable to form a filter having specifically defined areas of flexibility to facilitate placement and/or removal of the device within the body. To impart such flexibility, some devices require additional manufacturing steps be taken to alter the dimensions or composition of the various components. In some cases, multiple components or materials are employed to impart flexibility to the device, requiring the use of additional joining processes such as soldering or bonding to assemble the components together.
More recent trends in the art have focused on the use of shape-memory alloys (SMA) and superelastic alloys to impart flexibility. Depending on the particular method of manufacturing, materials such as nickel-titanium alloy (Nitinol) have the ability to return to a particular shape upon a temperature-induced phase transformation, or exhibit superelasticity when subjected to a stress-induced phase transformation. In contrast to more conventional metals such as stainless steel, shape-memory and superelastic alloys are able to endure greater strains before plastically deforming, allowing the material to be used in applications demanding greater flexibility and torqueability.
Linear elastic materials are typically formed of superelastic alloys that have been specially treated to maintain a linear stress-strain response. Although similar in chemical composition to their superelastic counterparts, linear elastic materials do not exhibit the flat stress-strain plateau generally associated with superelastic materials. Instead, as recoverable strain increases, the stress in the material continues to increase linearly until plastic (i.e. permanent) deformation is achieved. As a result, such materials tend to be axially and torsionally stiffer than superelastic materials at higher deformations.
While shape-memory and superelastic alloys have widespread applications in the medical industry, the manufacturing and processing steps required to incorporate such materials into medical devices have proven difficult. Nickel-titanium alloys are often difficult to solder or braze to other materials, due to the formation of TiO2 and other titanium oxides. Welding is difficult due to the formation of intermetallic phases. Machining has also proven difficult due to the rapid work hardening nature and the abrasiveness of the metal. In order to promote applications of nickel-titanium alloys in the medical device arena, processes to enhance site specific properties such as flexibility in a small region of a device must be developed.